Thermal sensitivities of respiration and protein synthesis differ among larval families of the Pacific oyster, Crassostrea gigas.

Understanding the mechanisms of biological responses to environmental change is a central theme in comparative and evolutionary physiology. Here, we analyzed variation in physiological responses to temperature, using 21 full-sibling larval families of the Pacific oyster, Crassostrea gigas. Pedigrees were confirmed with genetic markers for adult broodstock obtained from our breeding program. From these 21 larval families, 41 determinations of thermal sensitivity (Q10 values) were assayed for larvae of different sizes. For respiration, thermal sensitivity was consistent within a larval family during growth, but showed significant differences among families. Different Q10 values were evident among 21 larval families, with family accounting for 87% of variation. Specifically, four larval families maintained an increased thermal sensitivity for respiration (Q10 of 3). This physiology would confer resilience to rising temperature by matching the increased energy demand of protein synthesis (Q10 of 3 previously reported). For protein synthesis, differences in Q10 values were also observed. Notably, a family was identified that had a decreased thermal sensitivity for protein synthesis (Q10 of 1.7 cf. Q10 of 3 for other families), conferring an optimal energy allocation with rising temperature. Different thermal sensitivities across families for respiration (energy supply) and protein synthesis (energy demand) were integrated into models of energy allocation at the whole-organism level. The outcome of these analyses provides insights into the physiological bases of optimal energy allocation with rising temperature. These transgenerational (egg-to-egg) experiments highlight approaches to dissect components of phenotypic variance to address long-standing questions of genetic adaptation and physiological resilience to environmental change.

Multiple stressors, allostasis and metabolic scaling in developing zebrafish.

Deoxygenation and warming affect adult fish physiology in all aquatic ecosystems, but how these stressors impact the energetics of sensitive developing stages is largely unknown. Addressing this knowledge gap, we investigated chronic and acute effects of two stressors (high temperature and hypoxia) in yolk-sac larval (48-168 hpf) zebrafish (Danio rerio) energy budgets measuring, oxygen consumption rate, growth rate (absolute and specific), % net conversion efficiency, net cost of growth and scaling relationships. Embryos and larvae were raised under four chronic treatments: (1) control (28°C and PO2 21 kPa, T28O21), (2) high temperature (31°C and PO2 21 kPa, T31O21), (3) hypoxia (28°C and PO2 11 kPa, T28TO11) and (4) high temperature and hypoxia (31°C and PO2 11 kPa, T31O11). From each chronic treatment, larvae were acutely exposed to the same combinations of stressors for 1 h in a respirometer. At hatching, larvae from chronic high temperature (T31O21 and T31O11) treatments were larger (higher dry mass and standard length) than controls (T28O21 and T28O11), but by the end of the yolk-sac stage, increased metabolic demands diverted energy away from growth, increasing net cost of growth and lowering % net conversion efficiency. Control metabolic scaling relationships were significant and differed from 0.75, but metabolic levels were lower in acute hypoxia and high temperature/hypoxia. Thus, high temperature dominated larval energetics, acting synergistically with hypoxia to increase cumulative energetic costs and making allostasis difficult compared with older stages.

Transgenerational plasticity and the capacity to adapt to low salinity in the eastern oyster, Crassostrea virginica.

Salinity conditions in oyster breeding grounds in the Gulf of Mexico are expected to drastically change due to increased precipitation from climate change and anthropogenic changes to local hydrology. We determined the capacity of the eastern oyster, Crassostrea virginica, to adapt via standing genetic variation or acclimate through transgenerational plasticity (TGP). We outplanted oysters to either a low- or medium-salinity site in Louisiana for 2 years. We then crossed adult parents using a North Carolina II breeding design, and measured body size and survival of larvae 5 dpf raised under low or ambient salinity. We found that TGP is unlikely to significantly contribute to low-salinity tolerance since we did not observe increased growth or survival in offspring reared in low salinity when their parents were also acclimated at a low-salinity site. However, we detected genetic variation for body size, with an estimated heritability of 0.68 ± 0.25 (95% CI). This suggests there is ample genetic variation for this trait to evolve, and that evolutionary adaptation is a possible mechanism through which oysters will persist with future declines in salinity. The results of this experiment provide valuable insights into successfully breeding low-salinity tolerance in this commercially important species.

Differing thermal sensitivities of physiological processes alter ATP allocation.

Changes in environmental temperature affect rate processes at all levels of biological organization. Yet the thermal sensitivity of specific physiological processes that affect allocation of the ATP pool within a species is less well understood. In this study of developmental stages of the Pacific oyster, Crassostrea gigas, thermal sensitivities were measured for growth, survivorship, protein synthesis, respiration and transport of amino acids and ions. At warmer temperatures, larvae grew faster but suffered increased mortality. An analysis of temperature sensitivity (Q10 values) revealed that protein synthesis, the major ATP-consuming process in larvae of C. gigas, is more sensitive to temperature change (Q10 value of 2.9±0.18) than metabolic rate (Q10 of 2.0±0.15). Ion transport by Na+/K+-ATPase measured in vivo has a Q10 value of 2.1±0.09. The corresponding value for glycine transport is 2.4±0.23. Differing thermal responses for protein synthesis and respiration result in a disproportional increase in the allocation of available ATP to protein synthesis with rising temperature. A bioenergetic model is presented illustrating how changes in growth and temperature affect allocation of the ATP pool. Over an environmentally relevant temperature range for this species, the proportion of the ATP pool allocated to protein synthesis increases from 35 to 65%. The greater energy demand to support protein synthesis with increasing temperature will compromise energy availability to support other essential physiological processes. Defining the trade-offs of ATP demand will provide insights into understanding the adaptive capacity of organisms to respond to various scenarios of environmental change.